As fossil fuel such as petroleum, coal and natural gas is being exhausted and problems caused by the fossil fuel such as environmental contamination and global warming get serious, it is urgently requested to develop clean energy source to replace the fossil fuel.
As an effort to develop alternative energy for fossil fuel, studies have been undergoing to convert natural energy such as solar heat, wind force and tidal power directly into electric power and to produce hydrogen energy from natural resources such as water.
Hydrogen energy is the most promising candidate for alternative energy because it uses water as a raw material which is most abundant on earth, it does not produce any pollutant during hydrogen combustion, suggesting that hydrogen energy is very clean energy, and it is also functioning as a storage medium for energy.
The representative methods for producing hydrogen from water are biological method, photochemical method, electrolysis, direct thermolysis and thermochemical degradation. Electrolysis, one of the conventional techniques, is in practical use, but others are still under study.
To decompose water directly to produce hydrogen, high temperature of at least 4000 K is required. So, direct decomposition of water is in fact very difficult. Therefore, water decomposition is tried stepwise, precisely a method is designed to contain chemical reactions induced stepwise at comparatively low temperatures to decompose water, which is a close cycle. That is the hydrogen production method by using thermochemical cycle.
Thermochemical cycle absorbs heat to convert it into hydrogen and oxygen chemically. This method produces hydrogen from water by multi-step reactions including oxidation and reduction of a metal oxide using heat.
Thermochemical cycle can be classified into pure thermochemical cycle and combined thermochemical cycle. The combined thermochemical cycle is developed to supplement and improve the pure thermochemical cycle, for which electrolysis or photochemical method is introduced.
According to the chemical reaction step, thermochemical cycle is classified as 2 step cycle, 3 step cycle, 4 step cycle, etc, and up to 8 step cycle has been notified so far.
Up to date, approximately 300 thermochemical cycles have been reported including 2 step thermochemical cycle using a metal oxide such as Fe, Mn, Zn, Co, Sn and WO3, ZnO/Zn, Fe3O4/FeO, CeO2/Ce2O3, and SnO2/Sn as a material for thermochemical process.
To increase reactivity and thermal stability at high temperature, a transition metal such as Ni and Co or a refractory such as alumina can be mixed or alloyed, particularly in those cycles of ZnO/Zn thermochemical cycle, Fe3O4/FeO thermochemical cycle, CeO2/Ce2O3 thermochemical cycle and SnO2/Sn thermochemical cycle.
(Zno/Zn Cycle)ZnO→Zn+½O22000° C.Zn+H2O→ZnO+H2400° C.
(Fe3O4/FeO cycle)Fe3O4→3FeO+½O22000° C.3FeO+H2O→Fe3O4+H2400° C.
(CeO2/Ce2O3 cycle)2CeO2→Ce2O3+½O22000° C.3Ce2O3+H2O→2CeO2+H2400° C.
(SnO2/Sn cycle)½SnO2→½Sn+½O22600° C.½Sn+H2O→½SnO2+H2400° C.
The above cycles require high reduction temperature of at least 2000° C. So, construction of composition for a reactor is difficult. The temperature of at least 2000° C. can be obtained by concentrating solar heat, however, it is very difficult to use that heat efficiently because of reflection.
There was a report on 2 step thermochemical cycle in relation to hydrogen production by water decomposition after thermal reduction at 1000-140° C. using Co-ferrite, Ni—Mn-ferrite and Ni-ferrite. However, the thermochemical cycle accompanies non-stoichiometric reaction that is the reduction of some of oxygen in M (Co or Ni, or Ni and Mn)-ferrite, resulting in low hydrogen production with the yield of up to 20 cc/g.
Therefore, thermochemical cycle that facilitates reduction at a low temperature to mass-produce hydrogen from water is required.